Over the past years, hydrogenated silicon nitride (SiNx:H) has found widespread application in the production of high-efficiency solar cells. This is due to its capability to accomplish several functions at the same time, which simplifies the fabrication process, for example.
In a typical commercial silicon solar cell production sequence, a thin layer of hydrogenated silicon nitride of about 100 nm is deposited on an n-p junction by a plasma-enhanced chemical vapor deposition process. The silicon nitride layer not only serves as an anti-reflection coating, but also effects an accumulation of positive charge at the SiNx:H—Si interface that aids in surface passivation. Furthermore, it introduces hydrogen into the silicon of the cell which resides within a thin plasma-damaged surface layer. Following the deposition of SiNx:H, metal contacts may be screen printed and fired through the anti-reflection coating. The metal hereby penetrates the coating to form a low-resistance ohmic contact, while the hydrogen diffuses into the bulk of the cell to passivate impurities and defects. The multi-purpose role of the hydrogenated nitride layer thus requires it to be a low-absorption anti-reflective coating, serve as a barrier for control in metallization, and promote favorable electronic processes that can passivate the surface as well as the bulk of the cell. It is therefore of vital importance that the deposition process of SiNx:H is designed carefully, so as to optimize the optical and electronic properties of the solar cell.
PECVD may be performed using a variety of techniques, such as, for example, Inductively Coupled Plasma (ICP) and Expanding Thermal Plasma (ETP). ETP-sources have proven to be an excellent choice for the deposition of thin, dense and high quality SiNx:H layers at deposition speeds of tens of nanometers per second. Research has revealed, however, that films that are deposited on substrates by straightforwardly moving them through the plasma plume of an ETP-source typically exhibit less than optimal optical and electronic characteristics.
It is therefore beneficial to provide an ETP-based thin film deposition assembly and a method that are capable of depositing layers, such as, for example, the aforementioned anti-reflective coatings, having improved optical and/or electronic properties.